1. Field of the Invention
The invention concerns dispersants based on copolymers with polyoxyalkenyl functional groups, processes for the production thereof and use thereof.
2. Description of the Related Art
Dispersants are added to mortar mixtures based on inorganic binders such as cement, limestone and gypsum in order to improve their workability, for example their fluidity. In order to make such mortar mixtures available in a workable consistency, significantly more mixing water is generally necessary than for the hardening or hydration step for the consolidation of the mortar. This excess water content, evaporating during the hardening, leads to a worsening of the mechanical strength of the construction compounds. To improve the consistency, in other words the flowability, of mortar mixtures with a given water-binder ratio, dispersants, called plasticizers, are added.
As plasticizers for flowable hydraulically setting mortar systems, substances based on ligninsulfonate are known from WO-A 97/13732. DE-A 19538821 describes sulfonate-containing condensation products based on amino-s-triazines with at least two amino groups and formaldehyde. In WO-A 00/75208, condensation products of sulfonated aromatics and formaldehyde are described. However, there are some ecological objections to such plasticizers, at least for indoor applications, owing to the liberation of formaldehyde.
Also known, for example from DE-A 19539460, are plasticizers for cementitious systems based on 2-methyl-2-acrylamidopropanesulfonic acid or other strong polyelectrolytes. These dispersants can as a rule only be used in combination with emulsifier-stabilized polymer dispersions. In combination with the polyvinyl alcohol-stabilized dispersions mainly used in the construction sector, these have the disadvantage that they result in massive destabilization of polymer dispersions (leading to coagulation) or the redispersion powders produced therefrom. If a mixture with polyvinyl alcohol-stabilized dispersions does not coagulate immediately, this mainly manifests itself in the fact that in use the cement mixtures already stiffen markedly during mixing, set prematurely or the required degree of spreading of hydraulically setting fillers is no longer attained. In addition, there is usually a marked shrinkage of the cement matrix, which can result in fissuring.
A further disadvantage of the previously mentioned plasticizers is the fact that the plasticizing action does not persist over a sufficiently long period. This results in a shortening of the working time, which becomes a problem when there is a longer period between the mixing of the construction material mixtures and the working of these.
So-called high performance plasticizers exhibit a longer-lasting plasticizing action. From EP-A 792850, cement compositions are known which contain as plasticizers copolymers of short-chain polyalkylene glycol (meth)acrylates, long-chain polyalkylene glycol (meth)acrylates) and ethylenically unsaturated carboxylic acids. EP-A 590983 concerns cement plasticizers based on copolymers of (meth)acrylic acid, ethylenically unsaturated sulfonates, polyethylene glycol (meth)allyl ethers, esters of (meth)acrylic acid with polyethylene glycol monoethers and optionally (meth)acrylate esters. In DE-A 10063291, cement plasticizers based on polycarboxylates are described, wherein comonomer units with polyoxyalkylene groups and comonomer units with OH—, CO— or sulfonate groups are still contained in the copolymer. EP-A 816298 describes plasticizers which are obtained by copolymerization of monomers with polyoxyethylene functional groups, polyoxyethylene-polyoxypropylene group-containing monomers and comonomers with ethylenically unsaturated sulfonate functional groups.
These plasticizers are characterized by longer-lasting plasticizing action, but tend to water separation (bleeding). This is attended by impaired working (distribution on the substrate to be smoothed) and poor self-healing.
Further, casein is used as a plasticizer in flowable hydraulically setting mortar systems. Casein provides unique flow, working and self-healing properties for flowable hydraulically setting mortar systems and in addition has binder characteristics. Casein is a milk protein, which is obtained by acid precipitation. It is characterized by major quality variations depending on the particular season and fodder quality. This renders its use in flowable hydraulically setting mortar systems more difficult. Furthermore, after working, casein-containing flowable hydraulically setting mortar systems tend to the formation of mould cultures, which is not desirable in living areas.
The problem was therefore to provide dispersants which display a long-lasting plasticizing action in cement systems, are compatible both with emulsifier and also with protective colloid stabilized systems, and exhibit the advantageous rheological properties of casein.